In electrophotography or similar imaging process, a latent image is formed on a photoreceptor and then developed by charged toner particles. The developed toner image is transferred to a receptive substrate and fixed on the substrate by heat. However, there are many issues related to the physical properties of the fused image. For example, the toner image can become sticky and tacky during storage, depending on the environment conditions. When the image is tacky, it may transfer portions of the image to the other side of adjacent substrates or images so that the images are damaged. This phenomenon is called blocking in many references. During double-side printing the fused image on one-side need to go through the fusing device again and the image quality of oneside may be negatively impacted during the second heat. The toner image may also become sticky during second heat and block to substrate and fusing device. In addition, the toner image may be damaged by physical abrasion and wear during application.
Those problems can be solved through radiation curable toner approach. In this approach, the heat-fused toner image is further cured or crosslinked by radiation, e.g. UV light. The mechanical properties of the toner image, e.g. toughness and glass transition, are improved by crosslinking. The application of radiation curable toners has been suggested in several patents, e.g. U.S. Pat. No. 5,905,012, No. 6,608,987, No. 5,470,683 and No. 6,535,711.
In the conventional method of making electrophotographic toner powders, a binder polymer and other ingredients, such as a pigment and a charge control agent, are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. The conventional grinding process has one significant drawback for the preparation of radiation curable toners. The radiation curable binders contain unsaturated functional groups, which are supposed to be crosslinked only upon radiation. However, the crosslinking reaction may also occur during melt blending or extrusion of the grinding process, when the materials are exposed to high temperature. This kind of side reaction not only destroys the functional groups for further radiation curing but also makes the toner materials tougher and more difficult for grinding. The crosslinking side-reaction also increases the glass transition temperature (Tg) of the radiation curable toner. Due to the side-reaction, it is difficult to control Tg of the final toner product, which impacts its fusing properties directly. In addition, uncontrolled crosslinking reaction creates inhomogeneity during melt blending or extrusion and the heterogeneous toner compounds result in a wide particle size distribution. Consequently, the yield of useful toner is lower and manufacturing cost is, therefore, higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
U.S. Pat. No. 5,470,683 prepares photosensitive microcapsule toners through emulsion polymerization. This kind of preparation process could minimize the crosslinking side-reaction because of its relatively low processing temperature and microcapsulation structures. However, the nature of the emulsions limits the toner binder variety and the manufacturing of the encapsulated toners is relatively complicated.
The present invention is related to the preparation of radiation curable toner particles by chemical toner technology, particularly, by evaporative limited coalescence process. In this process, toner particles are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer such as colloidal silica, and removing the solvent by evaporation. The resultant polymer particles are then isolated, washed and dried. During the evaporative limited coalescence process no heat or a limited amount of heat is used and the toner binders do not need to be exposed to any temperature significantly higher than room temperature. Those process conditions prevent the crosslinking side reaction of the unsaturated functional groups of the radiation curable materials.
The evaporative limited coalescence offers many other advantages over the conventional grinding method of producing toner particles. Toner particles can be prepared by the evaporative limited coalescence technique from any type of binder polymer that is soluble in a solvent that is immiscible with water. The size and size distribution of the resulting polymer particles can be controlled by the relative quantities of the polymer employed, the solvent, the quantity and size of the water-insoluble particulate suspension stabilizer and the size to which the solvent-polymer droplets are reduced by the agitation employed in dispersing the organic solution in the aqueous medium. Representative patents disclosing toner manufacture by limited coalescence and advantages thereof include U.S. Pat. Nos. 4,833,060, 4,835,084, 4,965,131, 5,133,992, 6,294,595 6,416,921 and 6,482,562, each of which is incorporated herein by reference.